A switched-mode power converter (also referred to as a “power converter” or “regulator”) is a power supply or power processing circuit that converts an input voltage waveform into a specified output voltage waveform. DC-DC power converters convert a direct current (“dc”) input voltage that may be derived from an alternating current (“ac”) source by rectification into a dc output voltage. Controllers associated with the power converters manage an operation thereof by controlling conduction periods of power switches employed therein. Some power converters include a controller coupled between an input and output of the power converter in a feedback loop configuration (also referred to as a “control loop” or “closed control loop”) to regulate an output characteristic of the power converter. Typically, the controller measures the output characteristic (e.g., an output voltage, an output current, or a combination of an output voltage and an output current) of the power converter, and based thereon modifies a duty cycle of a power switch of the power converter to regulate the output characteristic. Other power converters operate in an open-loop manner wherein an output voltage is produced substantially proportional to an input voltage.
A power converter with a low power rating designed to convert an ac mains voltage to a dc output voltage to power an electronic load such as a printer, modem, or personal computer is generally referred to as an “ac power adapter” or a “power adapter,” or, herein succinctly, as an “adapter.” Industry standards and market needs have necessitated continual reductions in no-load and low-load power supply loss to reduce power consumed by millions of power adapters that may remain plugged in, but are not in use, or that may supply a light load level to an electronic device that is not operating at its full capacity. Efficiency requirements at low output power levels or in an idle state have become important in view of the typical load presented by an electronic device in an idle or sleep mode, or an electronic device not operating at full capacity, which are common operational states for a large fraction of the time for electronic devices such as computers and printers in a home or office environment.
To initiate the switching action of a power converter, the controller needs a source of current at a startup current level (e.g., a few milliamperes) or at a bias voltage level (e.g., 12 volts). To provide this input power at startup, a startup circuit typically draws a few milliamperes from the input power source, which may be 140 volts or more. Thus, the input power required to start the power converter may approach 0.5 watt or more, which represents a significant level of power dissipation. The startup current increases the no-load input power if the startup current is not switched off after startup. To switch off the startup current, a high voltage switch is required, which can be relatively expensive.
Thus, light-load and no-load power losses, while relatively small, have now become substantial hindrances to improving power converter efficiency as industry requirements become stricter each year. Thus, despite the development of numerous strategies to reduce power losses of power adapters, no satisfactory strategy has emerged to provide substantial reduction of power dissipation while the adapter provides minimal or no power to a load. Accordingly, what is needed in the art is a design approach and related method for a power converter (e.g., a power adapter) that enable further reduction of power converter losses without compromising product performance, and that can be advantageously adapted to high-volume manufacturing techniques.